The "thick layer" technology is in very wide use in microelectronics for manufacturing interconnection substrates of hybrid circuits. These substrates serve as supports and as interconnection elements for the various electronic components constituting the circuit and, for this purpose, they are essentially formed by a mechanical support also performing the function of a thermal dissipator on which conductive, insulating and/or resistive layers are added which realize, with varied patterns, all of the interconnections. The substrate is usually a ceramic and most often 96% pure alumina.
The thick layers are formed with inks or pastes which are essentially constituted by a functional phase, an inorganic binder, and an organic vehicle (solvents and polymer resins which impart a good rheology to the paste).
The functional phase provides the electrical properties of the layer as concerns conductivity, resisitivity or insulation. It is in the form of powders which are metallic for the conductive layers, based on glasses or ceramics for the insulating layers. The inorganic binder has for function to ensure a good adherence between the layer and the support.
In practice, the deposition of a layer of raw ink on the substrate is carried out by serigraphy according to a pattern previously formed on a screen which is a sieve, and the deposit is obtained by the passage of the ink through the meshes of the screen: the drying operation is intended to eliminate the solvents, while the firing of the assembly at high temperature produces, after elimination of the polymer resins, by sintering and/or fusion of the constituents, a film having a thickness of between about 5 microns and about 50 microns which adheres to the surface of the support.
The firing of the layers of ink after drying represents a complex heat treatment operation in the course of which the role of the atmosphere is primary. In particular, the atmosphere provides an environment suitable for a carrying out of the operations for effecting the sintering and the adherence of the layer on the support which is as correct as possible.
In the case of conductive inks based on noble metals such as platinum, gold, palladium or silver, and with respect to the compatible insulating and resisitive inks, the firing is carried out under air. The use of an oxidizing atmosphere such as air is quite suitable for the operation eliminating the polymer resins remaining in the layer after drying. The oxygen present in the atmosphere facilitates this elimination of the resins by oxidation of the organic compounds which are vapourized and pyrolized with rise in temperature.
On the other hand, in the case of conductive inks based on a non-noble metal such as copper or other so-called compatible materials, the firing requires an inert atmosphere obtained by the use of neutral gases such as nitrogen, argon, helium, if desired a combination of these gases, so as to avoid the oxidation of the metal.
As concerns these conductive inks based on copper, it is recommended to effect the firing under nitrogen with less than 10 ppm of oxygen present in the atmosphere of the whole of the furnace. There may be cited in this respect as an example the technical notices for the use of copper inks issued by the firm Du Pont de Nemours which recommends effecting the firing of conductive copper pastes in an atmosphere of nitrogen containing between 5 and 10 ppm of oxygen.
In such an atmosphere, notwithstanding the fact that it contains a few ppm of oxygen, the elimination of the polymer resins is often incomplete, above all when large volumes of paste are treated or in the case of the production of multilayers which surround large areas of dielectric. As a result, there is a certain number of deteriorations of the layers and in particular a reduction in their adherence to the substrate. The mechanisms involved in these deteriorations were studied in the case of the deposition of copper inks on alumina, and are in particular described by R. J. Bacher and V. P. Siuta in an article entitled "Firing Process-related Failure Mechanisms in Thick-Film Copper Multilayers" which appeared in "Proc. of the Electronic Components Conference, IEEE," 1986 p 471-480.
One solution proposed concerns the preferential doping of the inert atmosphere of the furnace with oxygen in the region of the zone for eliminating the polymer resins without modifying the oxygen contents of the sintering and cooling zones. The authors of the article showed that the oxygen content, even with the zone for eliminating the polymer resins, must remain limited, in any case lower than 100 ppm, if satisfactory results are to be obtained. As an example, F. Franconville and M. Auray, in an article entitled "Copper displaces gold in production of multilayer substrates for computer application" which appeared in "Proc. of the Third European Hybrid Microelectronics Conference", 1981 p 174-187 proposed conditions of firing copper multilayers on alumina, recommending for the firing of compatible copper dielectric inks the presence of 150 to 300 ppm of oxygen in the zone for eliminating the polymer resins, however, noting that the firing of the copper conductive layers must be effected under nitrogen containing less than 20 ppm of oxygen in all the stages of the firing.
These recommendations therefore imply the use of two different atmospheres, one for the firing of the copper conductive layers and the other for the firing of the compatible copper dielectric layers.
Furthermore, in the aforementioned article by R. J. Bacher and V. P. Siuta, the authors also studied the effects of doping the nitrogen with oxygen in the zone of the elimination of the polymer resins on the properties of copper multilayer circuits obtained in these conditions and reached the conclusion that, for an oxygen content as high as 100 ppm, the hermetic property of the dielectric layers is largely improved. On the other hand, as concerns the copper conductive layers, these authors note a distinct degradation of the solderability of the copper layers in respect of oxygen contents of 100 ppm and higher contents. They indicate correct conditions of solderability for 30 ppm of oxygen in the polymer binder eliminating zone.
Lastly, E. A. Webb, in an article entitled "Effects of Copper Thick-Film Processing on Adhesion and Bondability" which appeared in "Proc. of the 6th European Microelectronics Conference, ISHM", 1987 Bournemouth, England p 128-135, studied the effect of the atmospheric conditions applied during the firing of copper conductive thick layers on the adhesion and solderability of the circuits obtained. This author found an improvement in the adhesion properties in the respect of oxygen contents in the polymer resin eliminating zone ranging from 7 to 15 ppm, whereas beyond 15 ppm of oxygen, the author notes a distinct degradation of the solderability of the copper layer.
The evolution of electronics toward the use of power circuits and/or the production of complex functions requiring a very dense implantation of the components, leads to a much greater dissipation of amounts of heat in the region of the substrate.
This evolution has resulted in a search for new electrically insulating materials possessing a thermal conductivity higher than that of alumina. The properties of beryllium oxide (BeO) make it a possible candidate, but its toxicity, when it is in the form of a powder, reduced its use in practice. On the other hand, aluminium nitride (AlN) possesses comparable electrical and thermal conduction properties but can be handled and machined without danger.
In this context, the copper/aluminium nitride system finally represents a definitely advantageous solution for the high integration of the components and for power microelectronics, in that it combines the exceptional properties of copper (high electrical conductivity, excellent solderability to tin, lead. . . ) and the high thermal conductivity of aluminium nitride.
The difficulty of obtaining quality copper thick layers on alumina was mentioned hereinbefore. The manufacture of thick copper layers on aluminium nitride presents further difficulties owing to the non-oxide character of this ceramic, which appears to be the cause of a low wetability of the aluminium nitride by the metals. In a technical communication ("Material Matters", Vol VII, No 3,) Electro-Science Laboratories presented a comparative study of the initial adhesion and after aging obtained in the case of thick copper layers produced on different sources of alumina and aluminium nitride. The adhesion was followed by the removal of the top layer of the copper studs of 4 sq. mm. Their results show that, with respect to all the tested substrates, the initial adhesions and the adhesions after aging measured in the case of an aluminium nitride substrate are about 50% weaker than the corresponding values obtained with respect to an alumina substrate.